Overcast days and humid conditions are ideal for powdery mildew, the fungal disease that coats leaves and stems of susceptible plants with dirty-white “fur.” In time, the unsightly fungus stunts growth and blooms. Though it may seem counterintuitive, spraying plants daily with a jet of water early in the day can reduce the spread by eliminating the dry surfaces required by the disease-carrying spores to germinate. On lightly infected plants, remove and dispose of infected leaves. Good air circulation between plants also helps keep the fungus at bay. Fungicides including non-toxic horticultural oils can slow mildew's spread but rarely eliminate it. In recent years, breeders have developed new varieties resistant to mildew and other diseases. Check labels of particularly sensitive plants like roses before you buy. Learn more about this and other fungal diseases at www.ipm.ucdavis.edu.